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<br />Basin Outlook Reports
<br />and
<br />Federal - State - Private
<br />Cooperative Snow Surveys
<br />
<br />For more water supply and resource management information, contact:
<br />Michael A. Gillespie
<br />Data Collection Office Supervisor
<br />USDA, Natural Resources Conservation Service
<br />655 Parfet St., Rm E200C
<br />Lakewood, CO 80215-5517
<br />Phone (720) 544-2852
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<br />How forecasts are made
<br />
<br />Most of the annual streamt10w in the western United States originates as snowfall that has accumulated in the mountains
<br />during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when
<br />it melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along
<br />with precipitation, antecedent strearnt1ow, and indices of the EI Nino / Southern Oscillation are used in computerized
<br />statistical and simulation models to prepare runoff forecasts. These forecasts are coordinated between hydrologists in the
<br />Natural Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are
<br />for t10ws that would occur naturally without any upstream influences.
<br />
<br />Forecasts of any kind, of course, are not perfect. Streamflow forecast uncertainty arises from thrce primary sources: (I)
<br />uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data.
<br />The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities
<br />of occurrence. The middle of the range is expressed b;1 the 50% exeeedance probability forecast, for which there is a
<br />50% chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value. To
<br />describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70%
<br />exceedance probability) and two larger values (30%, and 10% exceedance probability), For example, there IS a 90%
<br />chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted
<br />similarly.
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<br />The wider the spread among these values, the more uncertain the forecast. As the season progresses, forecasts become
<br />more accurate, primarily because a greater portion of the future weather conditions bccome known; this is reflected by a
<br />narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into
<br />consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing
<br />to assume about the amount of water to be expected. Ifusers anticipate receiving a lesser supply of water, or if they wish
<br />to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions
<br />on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned
<br />about receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or
<br />10% exceedance probability forecasts, or something in between. Regardless oflhe forecast value users choose for
<br />operations, they should be prepared to deal with enher more or less water. (Users should remember that even if the 90%
<br />exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using thc
<br />exceedance probability information, users can easily determine the chances ofrecei\"ing more or less water.
<br />
<br />The United Stales Departmen! of Agriculture (USDA) prohibits discrimination in all its programs on the basis of race, color, national ongin, gender,
<br />r~1igi~~,. age, disabill.ty. political beliefs, sexual orientalion and marital or family status. (Not all prohibited bases apply to aU programs.) Persons with
<br />disabilities who reqUire alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA's
<br />TARGET Center at 202-720-2600 (voice and TOO).
<br />
<br />To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326.W, Whitten Building, 14th & Independence Avenue, SW,
<br />Washington, DC, 20250-9410, or call (202) 720-5964 (voice and TOO). USDA is an equal opportunity provider and employer.
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